Color correction and modification has been in widespread use in connection with television advertisements and various aspects of film to tape transfer including preservation and restoration of color prints of deteriorating film media. Very sophisticated apparatus for selecting signal levels representing particular hues, and combinations of hue and saturation, in video signals have been developed. For example, color correcting apparatus is shown in U.S. Pat. Nos. 4,642,632; 4,727,412; 4,876,589; and 5,253,043.
Color correction and modification is used in a number of applications within the television industry and other businesses which make use of video and other graphic image signals. A principal use is in film to tape transfers and postproduction processing of commercials to highlight certain objects, colors, and the like. For example, when it is desired to emphasize a particular object in a scene or image, wherein the object has a detectable hue that is distinguishable from the hues of other objects in the scene (e.g. a spot color), the occurrence of picture elements ("pixels") containing this hue can be detected and the saturation level can be increased. In conventional color correctors, this has the effect of "highlighting" the particular object. For example, the hue distinctive to a red soft drink can in an advertisement can be saturated so as to draw additional attention of the viewer to the can.
For video equipment, a color correction system operator must typically go through a two-stage process when importing video images for color correction utilizing conventional color correction equipment. The first step typically involves adjusting controls at the video image source, for example, a telecine, so as to provide a wide dynamic range. The second step involves adjusting controls with the color correction equipment on a scene by scene basis.
In the first step, it is important to set up the video image source to provide a wide dynamic range of signal levels prior to their input at the color correction equipment. For example, if the video signals input have a dynamic range from 0-65,535 values (indicating 16 bits of digitization), but the predominant area of interest of a picture is concentrated within a range of, say, about 5,000 to about 30,000, then the operator would wish to scale the video signals from the image source so that the 5,000-30,000 range expands to fit the available dynamic range of the color correction equipment. The operator employs controls at the video image source equipment, e.g., gain, gamma, and black level, to scale the information to the dynamic range of the color corrector.
During this set up process, a color correction operator will often look at a vectorscope or waveform monitor to observe the effects of adjusting the controls at the image source. The operator typically must run a series of frames of source image while observing the vectorscope or waveform monitor, make adjustments, and then rerun the selected scene until satisfied that the signals representing the pictures have been scaled properly to fit the dynamic range of the color correction equipment.
With telecine type video image source equipment, adjustments are typically made by manually adjusting telecine photoelectric cells or the color controls GAIN, BLACK, and GAMMA for each of the red, green, and blue video channels to obtain the suitable dynamic range in overall signal level during set up. Then, the correction operator must adjust the color correction equipment controls, and sometimes the telecine's controls, to achieve a desired color balance on a scene by scene basis.
Even if a desired color balance is fixed, such as for neutral gray, flesh tones, etc., which are restricted to a narrow range of acceptable values, for many scenes the operator must manually and repetitively adjust each control until the appropriate color balance is achieved. These adjustments require a lot of repetitive, tedious work, both at the color correction equipment and sometimes to the image source equipment.
The need for the present invention occurred to the inventor during observation of the process of set up for the first stage of color correction. If a system could be devised to employ a computer or workstation to adjust video signal parameters, e.g. GAIN, BLACK and GAMMA, for each of the three color channels, the overall color correction process could be made more efficient.
Those skilled in the art will understand that adjustment parameters for the three primary color video signals red (R), green (G), and blue (B) are typically those of GAIN, GAMMA, and BLACK. Intuitively, GAIN is a factor that controls the overall amplification of the signal, and is most often associated with setting the peak amplitude. The BLACK level, on the other hand, is most often associated with the signal levels at the lower end of the scale, with nominal values (e.g. 16.sub.10) for each of R, G and B indicating minimal signal level and a completely black screen. Raising R, G, and B equally for low signal levels brings up the overall "BLACK" level in terms of shades of gray.
The GAMMA parameter, which originated in photography, relates to non-linear characteristics in the mid-range portion between the lowest signal levels and highest signal levels, and allows introduction of an approximation of the non-linear response characteristics of the human eye to light intensity levels. GAMMA correction intuitively has the effect of compressing or spreading out signal intensities in the middle range of intensity values perceptible by the human eye viewing a video screen. The GAMMA level of a video image is often associated with the most predominant signal level of a given picture, which can be measured by determining the signal level associated with the greatest number of pixels in the picture. frequency distribution. This is often referred to as the "peak" value, since a frequency distribution (histogram) of signal level vs. number of pixels will have a peak at this particular signal level.
Those skilled in the art will also understand that other video parameters and signal standards are commonly encountered in color correction equipment. The standard NTSC video signal represents color information in terms of hue, saturation, and luminance. Yet other types of video equipment provide color difference signals R-Y, B-Y, G-Y, where Y is a luminance signal determined from a matrix of R, G and B with weighted RGB values. As is well known to those skilled in the art, the NTSC standard broadcast television signal was adopted in the United States in 1941 and popularized shortly after World War II. In 1953, the current NTSC standard for color television broadcasting including a 3.58 MHz subcarrier carrying the chroma information, which composite signal was compatible with NTSC monochrome receivers, was adopted. Since continental Europe was recovering from the effects of World War II, it was somewhat later in adopting standard television signals. Most of Europe adopted a standard Phase Alteration by Line (PAL) composite broadcast signal with better resolution than that of the NTSC format. The French and certain others adopted a color system known as SECAM.
In recent years, attention of the television industries throughout the world has been turned to various proposals and apparatus for providing high definition television (HDTV) with both digital and analog composite signal methods. Many of the current processing techniques for video signal correction are carried out in the digital domain. In order to promote the international interchange of video signals and to standardize the interface between digital video signal sources and devices utilizing or transmitting same, the International Radio Consultant Committee (CCIR) promulgated Recommendation No. 601-1 in 1986 which defines a standard set of digitized color signals for television studios. CCIR Recommendation 601-1 (1986) is hereby incorporated by reference.
It is within the scope of the prior art to take CCIR 601-1 digitized signal streams, convert same to corresponding RGB signals (in analog or digital domains), and then to perform the primary and secondary color corrections and modifications on the resultant signals. The corrected or modified signal can then be passed through a signal matrix and reconverted to a digital bit stream through use of an analog to digital converter. The standard signals defined in Recommendation 601-1 essentially consist of a luminance signal Y and two color difference signals (R-Y) and (B-Y). It is well known that, since the luminance signal contains information on levels of red, green and blue (RGB), the three standard signals can be used to reproduce the RGB levels for any given set of samples.
The need for the present invention occurs not only in equipment for analog video signal processing, but also in equipment for HDTV video signal processing, digital video signal processing, postproduction color correction equipment, and broadcast television. Moreover, there is need for the invention in other color graphics/image processing technologies such as graphics generating devices and displays, print media, digital photography, etc.